Most practical television systems comprise the dissection and reconstruction of an image that comprises a plurality of picture elements, or pixels. Dissection of the image is performed by an optoelectronic transducer that comprises an image sensor. An image sensor has, or simulates, a plurality of discrete sensor pixel elements arranged to allow each sensor pixel element to provide a brightness value that corresponds to the average brightness of a corresponding image pixel. The brightness values of the sensor pixels are sequentially transmitted as a sequential image signal over a single transmission channel to avoid the complexity of providing a separate transmission channel for each pixel.
The image is reconstructed by sequential distribution of the sequential image signal within an optoelectronic transducer that comprises an image display. An image display comprises, or simulates, a plurality of image display pixels, so that the brightness value of each sensor pixel controls the brightness level of a corresponding display pixel. If the image sensor or display comprises an array of truly discrete pixels, the pixels must be individually addressed as part of the image dissection or reconstruction process.
Methods and apparatus for individually addressing discrete pixels of image sensors and displays have attempted to reduce the inherent complexity involved in individually selecting each pixel in the array according to a predetermined sequence at a predetermined scanning rate. The earliest attempts to address an image display comprising an array of pixels, typically an array of neon glow lamps, involved addressing each of the glow lamps by sequentially switching the image signal to each glow lamp with a motor-driven commutator.
Of course, with arrays that comprise hundreds of thousands of discrete pixels, such a motor-driven commutator is not practical because the commutator would have to have at least one contact for each pixel. It would not only be very expensive, but the large number of contacts and associated connecting wires would result in serious image degradation due to variation of impedance over the frequency range of the image signal.
One relatively simple way to address, or scan, each pixel in a group of pixels is to make each pixel sensitive to a scanning signal, such as a pulse, wherein each pixel is responsive upon receipt of the scanning signal only for the duration of the pulse. If the scanning signal is propagated from one pixel to another at a predetermined pixel scanning rate, then each pixel in the group will sequentially become responsive in the proper sequence for the proper duration.
A later attempt to reduce the number of contacts in the mechanical commutator for an image display involved the use of such a pulse-like scanning signal. This system involved switching the sequential image signal from one line of pixels to another with a mechanical commutator, but turning on each display pixel in each line with the scanning signal. The scanning signal was propagated along a passive delay line, with each pixel in each line of pixels connected at a different point along the delay line so that each pixel would turn on at the proper instant for the proper duration.
Later attempts to sequentially address image display pixels eliminated mechanical switching by making each pixel sensitive only to the receipt of a pair of scanning signals, with a first scanning signal propagated from one line of pixels to another at a line scanning rate through a first delay line and a second scanning signal propagated from one pixel to another in each line at a pixel scanning rate through a second delay line. These various attempts involved combining the image signal with the line or pixel scanning signals so that the image signal would control the brightness of each pixel made responsive by the receipt of the two scanning signals.
These systems suffer from at least two serious problems when used for image displays that comprise a large number of pixels. First, the amplitude and waveform of the pulse-like scanning signal deteriorates as it travels down the delay lines, with the deterioration proportional to the number of delay line sections. Since at least one section is required for each line of pixels in the first delay line and at least one section for each pixel in a line of pixels in the second delay line, serious scanning signal deterioration is likely for dense arrays. This can lead to erratic scanning or brightness levels in at least a portion of the array.
Second, all prior systems have either required switching or combining of the image signal through the individual address lines for the lines of pixels or for the pixels in each line. This increases the likelihood of bleed-over of the image signal and scanning signal to adjacent address lines, thereby causing more than one pixel to become responsive at any instant. The wide-band image signal is also degraded by propagation through the address lines due to the inductive and capacitive reactances associated with the long, thin and closely coupled address lines required for dense arrays.
An early system used a complex arrangement of impedance networks in the delay line in an attempt to reduce degradation of the scanning signal waveform as the scanning signal propagated down the delay line. The use of delay lines with complex sections would make their implementation with dense arrays prohibitively expensive. Furthermore, the attenuation of such lines is usually greater than simple delay line configurations.
Another system compensated for the amplitude loss of the scan pulse as it was propagated through each delay line with the use of resistances of varying values at each tap in the delay line to reduce amplitude variation. As a result, the amplitude of the scanning signal generated for dense pixel arrays would have to be very large. Also, the resulting shift in load resistance at each tap would cause deterioration of the scanning signal waveform as it propagated down the delay line.
Because of the problems associated with the propagation of scanning signals to pixels through delay lines as described above, much effort has been directed to the use of binary counters or shift registers for activation of pixels in an array through similar addressing schemes. For instance, a first shift register system would be used to sequentially provide a first activation signal to each line of pixels and a second shift register system would be used to sequentially provide a second activation signal to each pixel in each line of pixels. In this case, each pixel is sensitive to the receipt of the combination of the first and second activation signals.
Although the use of shift registers provide a robust activation signal for each pixel in the array during the scanning process, the complexity of the circuitry for dense arrays is undesirable. Furthermore, the shift registers must be driven by suitable clock circuitry, and the clock circuitry must be synchronized to the image signal with special synchronization circuitry. The implementation of a scanning system for dense arrays using such shift registers would therefore be complex and costly.
The systems that have used such shift registers for the scanning process have all involved the switching or combining of the image signal with the activation signals to propagate the image signal down at least one set of address lines. This causes deterioration of the wide-band image signal required for dense arrays, and also creates the possibility of bleed-over of the image signal and scanning signal to adjacent address lines, thereby causing more than one pixel to become responsive at any instant, as described above for prior art delay-line systems.